Cell balancing can be used for battery packs including multiple cells. If the battery cells are not balanced, the battery packs may have less available capacity, since the capacity of the weakest cell in the serially connected cells can determine the overall pack capacity. Unbalancing can occur when a difference in cell voltages is greater than a certain level. In battery systems, battery cell information can be measured by a circuit on-chip or a printed circuit board (PCB). A battery management board can balance cells according to the measured battery cell information. In relatively large battery management systems which include many battery modules, battery cell balancing can be implemented in each battery module.
FIG. 1 illustrates a block diagram showing a conventional system 100 for balancing battery cells. The system 100 includes battery modules 182 and 184 having similar structures. The battery module 182 includes battery cells 102 and 104, cell balancing circuits 122 and 124, cell voltage sampling circuits 132, 134, 136 and 138, a module balancing circuit 112, and a battery management module 162. The battery module 184 includes battery cells 106 and 108, cell balancing circuits 126 and 128, cell voltage sampling circuits 142, 144, 146 and 148, a module balancing circuit 114, and a battery management module 166.
The cell voltage sampling circuits 132, 134, 136 and 138 monitor voltages across the battery cells 102 and 104. The battery management module 162 acquires the sampled voltages from the cell voltage sampling circuits 132, 134, 136 and 138, selects a reference cell voltage (e.g., a minimum voltage) from the sampled voltages, and compares the reference cell voltage with the sampled cell voltages respectively. If a difference between the reference cell voltage and the cell voltage of a battery cell, e.g., the battery cell 102 or 104, is greater than a predetermined threshold, the battery management module 162 can control the cell balancing circuits to bypass a current of the battery cell. As such, voltages of the battery cells 102 and 104 can be balanced within the battery module 182 according to the reference cell voltage. The voltages of the battery cells 106 and 108 can be balanced in a similar manner.
In the system 100, the battery modules 182 and 184 perform cell balancing within each individual module. If the unbalance occurs between the battery modules 182 and 184, the module balance circuits 112 and 114 are used for bypassing the current of the entire battery module. However, the efficiency of the battery system can be reduced. Moreover, since the current flowing through the entire battery module is bypassed for cell balancing, power consumption can be increased.